This invention relates to an injector for use in supplying fuel to a cylinder of an internal combustion engine.
In order to reduce the combustion noise and emissions levels of an engine, it is desirable to supply each cylinder of the engine with a relatively small quantity of fuel followed by a main injection during which most of the fuel is supplied to the cylinder. The fuel may be supplied either by supplying two separate injections, a pilot injection followed by a main injection, or alternatively, the injector may be arranged to supply fuel at an initial, low rate, subsequently supplying, fuel at a higher rate during each injection.
A number of two-rate injectors are known in which a restriction is defined between a needle of the injector and the wall defining a bore within which the needle is slidable. In use, when the needle is lifted from its seating by a small amount, the restriction acts to limit the rate at which fuel is supplied towards the seating, and hence the injection rate. Subsequently, the needle is lifted from its seating by a greater amount, such movement of the needle increasing the flow area through the restriction to a sufficient extent that the fuel flow therethrough is substantially unrestricted, hence permitting fuel to flow towards the seating at an increased rate, thus permitting the injection rate to increase.
In such two-rate injectors, in order to control the injection rate, the rate of lifting of the injection needle away from its seating needs to be accurately controlled, and such control is difficult to achieve consistently.
It is an object of the invention to provide a two-rate injector of relatively simple construction.